This invention relates to a time-division multiple pulse code modulation system, and more particularly it relates to a digital transmission system which interleaves auxiliary signals in a multiplexed data bit stream, and also converts them to bit sequence independent line codes. The present invention is applicable to optical digital transmission systems.
The wide band characteristic of optical fiber cables has made it possible to employ nBmB (m&gt;n) DC balanced codes (for instance, 5B6B codes and 6B8B codes) in line codes. Reference for such a proposal is made to, for example, "A 140 Mbit/s Optical Transmission System with 8 km Repeater Spacing and Line Section Length of 96 km", by K. Mouthaan and J. R. Schlechte, Conference Proceedings of Optical Communication Conference, Amsterdam, Sept. 17-19, 1979. However, this proposal merely suggests conversion to 5B6B line codes, but failed to mention anything about the service signal transmission, such as order wire and alarm transmission, required in the terminal repeater system. Since nBmB codes are block codes consisting of m bit words, it is necessary to synchronize words on the receiver side, thereby insuring synchronism. In the conventional optical digital transmission system, it would be most uneconomical to provide a separate electric transmission line for auxiliary signals.